The Basel astronomer Tammann made significant contributions to cosmology. After studying in Basel and Göttingen, he worked at the Mount Wilson and Mount Palomar Obseratories in California and started a long-lasting collaboration with the American astronomer Allan Sandage. Together, they made significant contributions to the determination of the cosmic distance scale in astronomy. Their subsequent measurements of the Hubble constant and determinations of the age of the universe played an important role in cosmology. From 1977 until his retirement Prof. Tammann was professor of astronomy at the Department of Physics and Astronomy at Basel University and director of the Astronomical Institute there. From 1980 to 1983 Gustav Tammann served as President of the Astronomische Gesellschaft, for the following three years his wa its Vice President.